Computer disk drives store information on magnetic disks. Typically, the information is stored on each disk in concentric tracks. The data tracks are usually divided into sectors. Information is written to and read from a disk by a transducer head. The transducer head may include a read head separate from a write head, or the read and write head may be integrated into a single read/write head. The transducer head is mounted on an actuator arm capable of moving the transducer head radially over the disk. Accordingly, the movement of the actuator arm allows the transducer head to access different data tracks. The disk is rotated by a spindle motor at a high speed, allowing the transducer head to access different sectors within each track on the disk.
The actuator arm is interconnected to a motor, such as a voice coil motor (VCM), to move the actuator arm such that the transducer head moves radially over the disk. Operation of the VCM is controlled by a servo control system. The servo control system generally performs two distinct functions: seek control and track following. The seek control function comprises controllably moving the actuator arm such that the transducer head is moved from an initial position to a target track position. In general, the seek function is initiated when a host computer associated with the computer disk drive issues a command to read data from or write data to a target track on the disk. Because of the increasingly high demands on the performance of computer storage devices such as disk drives, it is desirable that the transducer head be moved from its initial position to a target track as quickly as possible. Once the transducer head has been moved sufficiently close to the target track by the seek function of the control system, the track following function of the servo control system is activated to center and maintain the transducer head on the target track until the desired data transfers are completed.
Typically, the transducer head will oscillate about the center line of the target track for a period of time following the transition of the servo control system from the seek mode to the track following mode. Because data written while the transducer head is oscillating about the centerline of a track may be unrecoverable during subsequent attempts to read that data, write operations are typically prohibited for a period of time following a transition from the seek mode to the track following mode. In addition, because data from adjacent tracks may inadvertently be read, or may corrupt the read signal collected by the transducer head during read operations attempted while the transducer head is oscillating, read operations are also typically inhibited for a period of time following a transition from the seek mode to the track following mode. By providing such “settling times” during which reading and writing by the transducer head is not allowed, the integrity of data written to or read from a disk drive may be better ensured.
Computer disk drives are also susceptible to data errors due to external shocks. This is because shocks can cause the transducer head to deviate from a desired position over the centerline of a data track. Therefore, it is important to prohibit the transfer of data to and from the disk during shock events. In particular, it is important to prohibit the writing of data to a disk when shock events occur, to prevent unrecoverable errors, such as may occur when data is written to unintended areas of the disk.
As will be understood by those of skill in the art, the “centerline” of a data track does not necessarily coincide with the physical centerline of the data track. Instead, “centerline” may refer to the center of the intended data storage area of the data track. Therefore, as used herein, “centerline” of a data track need not refer to the physical centerline of the data track, and “centered” indicates that the transducer head is properly centered over the area within the track that is intended for data storage, regardless of whether that position coincides with the physical centerline of the data track.
A track misregistration error occurs when the transducer head of a disk drive is not properly centered over a data track. In particular, a read track misregistration error occurs when the read head of a transducer head is not properly centered over a data track centerline. Likewise, a write track misregistration error occurs when a write head of a transducer head is not properly centered over a data track centerline. Write track misregistration errors are particularly troublesome, because they can result in permanent data loss. For instance, data written to the disk while the write head is not centered over the data track may be unrecoverable during read operations performed later to retrieve the previously written data. This is because the read head, traveling over the centerline of the data track (i.e., looking for the data in the expected position), may not be able to retrieve the data that was written off-center. In addition to the loss of the data being written, data previously written to adjacent tracks may also be lost. For example, data written while a write head is off center may completely overwrite data in an adjacent track, or may adversely affect the magnetic transitions storing the data in the adjacent track such that the previously written data is unrecoverable. For all of these reasons, it is important to detect track misregistration errors, and to prohibit writing by the transducer head while the transducer head is not properly centered over the target track.
Generally, a write fault occurs when a transducer head of a computer disk drive has deviated a predetermined distance from the centerline of the target track and the servo control system is in track following mode. Write faults are commonly caused by external shocks to the hard disk drive. In response to a write fault, a disk drive may trigger write fault condition which is maintained for a predetermined period of time, to allow oscillations caused by the shock event to dampen and disappear. While the write fault conditions is in effect, write operations are disabled.
In setting the distance that a transducer head must deviate from the track centerline (i.e., the magnitude of the tracking error) before a write fault is triggered, and in setting the amount of time during which the write function will be prohibited, consideration must be given to the data transfer performance of the disk drive. This is because delays in writing, although effective at avoiding track misregistration errors, reduce the data transfer performance of the disk drive. However, although the data transfer performance is of great concern to the designer of a computer disk drive, ensuring the integrity of data stored on the disk drive is of paramount importance.
Previous methods of detecting shock events and triggering write fault events have used accelerometers and other devices not required to perform the basic functions of a disk drive. Accordingly, such methods of detecting shock events add to the cost of the hard drive. In addition, previous shock detection methods treated all shock events equally, regardless of the severity of the shock event, and therefore unnecessarily compromised the data throughput performance and resistance to data loss of the hard drive.
It would be advantageous to provide a computer disk drive that is capable of preventing data misregistration errors due to shock events. In particular, it would be advantageous to provide a computer disk drive having a seek control system capable of reacting to shock events in different ways, depending on the severity of a particular event. In addition, it would be advantageous to provide a computer disk drive capable of registering the severity of shock events without the need for a separately provided shock detector. Furthermore, it would be advantageous to provide a computer disk drive capable of protecting against track misregistration errors without unduly limiting the data transfer performance of the disk drive.